Where to start?
You’re starting at the very beginning what are you looking to gain? what kind of target market are you after? and so on ask yourself these questions and then go do some Keyword Research both offline and online here are some places to go to.

Offline Keyword Research:
Brainstorming
Marketing Collateral
Customer Surveys
Listening to Customers

Online Keyword Research:
Google Autocomplete
Google Adwords
Keyword Planner
Google Trends
Keywords need to be relevant
When you are looking over keywords be sure to keep them relevant and have a direct connection to your content. Be sure to also accurately represent your product/service to your audience, the reason your audience is coming to your website should be on your website. Look at your website what are the main things that pop out from everything, what is your website about?
Keywords need to generate enough search traffic
There are so many things we can look at while we’re looking at traffic, you want to make sure there is a search demand, make sure there is actual traffic for your keyword, and make sure people are actually searching these words. Google autocomplete is a good place to look, but it will not give you the best results, google is built specifically for each person what works for you won’t work for everyone. You also want to look at the Keyword Planner just to get some words that may work better at certain times. Don’t forget to keep up with your keyword trends you can always change your keywords over time. You want to keep up with what words work best for you but you also want to keep up with when these words work best for you, is it a seasonal thing or is it a time thing always look at that. You’re going to want to keep asking yourself these questions for your site.
Keywords ideally have low competition
Now, this is also a difficult part, your competition, you want to look at how many of your big competitors are using the same keywords.
Long-tail Keywords, are multi-word phrases, typically less used by competitors, but have a lower search volume
Fathead Keyword is one core term as a keyword
Using the Keyword Planner is a good tool, to view what words have less competition
Be sure to use all your tools available.
Optimization
This is the process of assigning keywords to specific pages on a website, it is also important to remember that the Google system is to rank the pages of a site and not the site as a whole.
When doing this process make sure you’re not overusing a keyword and have each page have its own word, it will avoid keyword cannibalization.
You will want broader keyword phrases for higher lever pages such as the homepage, Your more specific keyword phrases should be for lower-level pages like your product pages.